Method of and apparatus for drying sheet materials by high-frequency electric fields



1949- w. P; COHOE arm. 2,459,622

METHOD OF AND APPARATUS FOR DYING SHEET IATERIALS BY HIGH-FREQUENCY ELECTRIC FIELDS Filed larch 18, 1944 2 Sheets-Shoat 1 ATTORN EYS w. P. COHOE z-rAL APP Jan. 18, 1949.

2,459,622 Abs 2 Sheets-Sheet 2 IE'I'HOD- OF AND ARATUS FOR DRYING SHEET IATBRI BY HIGH-FREQUENCY ELECTRIC FIELDS Filed larch 18, 1944 4 m 4 6 4 y i w 1: m 7 u m W 3 f V 0w x :0 M ii! J J L I n w l. T |:l a: T MW W n 1 I WV J r J I I I n J 3 2 M u J H y 4 J. L z T u z J MN 2 m I w m 7 w x w m nu 4% m a n 7% Patented Jan. 18, 1949 METHOD OF AND APPARATUS FOR DRYING SHEET MATERIALS BY HIGH-FREQUENCY ELECTRIC FIELDS Wallace P. Cohoe, Riverdale-on-Hudson, and Mitchell Wilson, New York, N. Y., asslgnors to Fred'k H. Lovey Co., Inc., New York, N. Y., a corporation of New York Application March 18, 1944, Serial No. 527,122

This invention relates to the drying of sheets, webs or films and more particularly to a method of and apparatus for utilizing electric fields produced by high frequency currents to effect a transient increase in the temperature of a moving body whereby moisture and volatile constituents are removed therefrom or chemical and physical changes promoted.

While the-invention will be described in detail with reference to the drying of printed impressions on a traveling web, it is not limited to this specific application. The invention may be utilized in drying films of ink of varying composition applied as printed impressions or otherwise. It is also applicable to the removal of moisture from paper and fabrics and to the drying of coating and finishing compositions applied to paper and fabrics, generally to the removal of vaporizable constituents of films of various kinds, the polymerization of heat setting resins, and oxidation of drying oils.

The use of an electric field to generate heat within various bodies has been suggested heretofore, and various proposals have been made with respect to the application of the principles involved. Except in certain applications to the drying of adhesives in the preparation of veneers, little of practical value has been accomplished heretofore. Many diflicult problems arise in connection with the application 01' the electric field to sheets, webs and films, and the solution of such problems has not been apparent.

It is the object of the present invention to afford a simple, practical and commercially applicable method and apparatus whereby sheets, webs and films may be subjected effectively to an electric field for the purposeof heating and thereby removing volatile constituents such as water and organic solvents.

Other objects and advantages of the invention will be apparent as it is better understood by reference to the following specification and the accompanying drawing, in which Fig. 1 is a diagrammatic representation of an application of the invention in the drying of printed impressions on a web;

Fig. 2 is a longitudinal section through a cylinder carrying the electrodes which are supplied with high frequency current to maintain the electric field;

Fig, 3 is a section on the line 3-3 of Fig. 2; and

Fig. 4 is an enlarged sectional view of one of the electrodes.

We have discovered that to secure the maxi- Claims. (Cl. 219-47) mum available heating effect in a sheet, web or film as it is passed through an electric field, unaccompanied by corona discharge, it is desirable to employ a multiplicity of electrodes so formed and spaced with respect to each other as to provide a minimum of dead space due to electrode surface and a maximum of field space between them. In other words, the area of the surface of the electrodes should be reduced to the minimum compatible with requisite strength and other conditions. A minimum efllcient gap between the electrodes afiords a maximum current utilization. Moreover, in order to secure parallelism between the axis of the web and the electric field, the sheet, web or film should be momentarily in actual contact with the electrodes while it is subjected to the electric field generated between them by the high frequency current impressed thereon. By causing the sheet,

web, or film to actually contact the electrodes, the region of greatest intensity of the electric fields which normally would be in a direct path between the respective pairs of electrodes is caused to deviate and to pass through the sheet, web, or film so that the maximum drying effect is obtained.

It is also desirable to preheat the sheet, web or film before it enters the field in order to avoid absorption by the sheet, web or fllm of heat generated in the field without accomplishing the purpose for which the field is applied. Preheating to temperatures from to 200 F. will not only save much of the energy consumed in maintaining the electric field but will also cut the drying time in half even though the boiling point of the vaporizable constituent may be as high as 400 F. or higher.

To understand this phenomenon, it is necessary to realize that the heat capacity of a web of paper is relatively enormous. If a web carrying, for example, a printed film of ink is subjected momentarily to the electric field, the temperature of the film tends to rise sharply, but in the meantime heat flows rapidly to the paper and the maximum desired temperature is not attained in the film. A long interval is thus required to evaporate the volatile constituent of the ink. In other words, the lineal speed of the-web must be reduced in order to attain the desired result. If, however, the paper is preheated even to relatively low temperatures, the temperature gradient between the paper and the ink film is reduced. Hence, faster drying is possible because the available high temperature heat is concentrated in the ink film. Obviously energy is saved because less of the heat produced by the electric field is drawn oil! to the paper.

Since the paper, if heated, loses heat rapidly to the surrounding atmosphere, I have found that maintenance of the temperature even during the passage of the sheet or web through the electric field is desirable. Thus the application of radiant heat to the under side of the sheet or web as it passes through the electric field ensures a minimum base temperature in the web and reduces the load on the electric field because less of the heat generated by the field is drawn into the paper. The heat produced by the efiect oi the electric field on the film is concentrated in the film which is to be dried.

For the purpose of clarity, we shall describe the details of the invention as applied in drying a printed paper web in a printing press. The same procedure may be applied in the treatment of coated paper, fabrics printed or coated, and

to films generally which include a volatilizable constituent or one which is hardened or otherwise modified by the application of heat. Many of the materials employed in printing inks and pastes and coating compositions are subject to heating in the electric field, and any such materials may be utilized for the practice of the invention.

Referring to Fig. i of the drawing, indicates a paper web which is fed to a printing mechanism of any suitable type. This is illustrated conventionally by a roller 6 which delivers the web to a packing cylinder i. A printed impression is applied to the web from a plate cylinder 8 which is inked by inking rolls 9. The details of the printing mechanism form no part of the present invention and may be modified in accordance with the customary practice.

From the packing cylinder i. the web 5 passes over a roller it which is supplied with heat in any suitable manner as for example by introducing steam to raise the temperature of the web to any desired point, for example from 175 to 200 F. The web so heated passes to a cylinder it where it is subjected to the electric field between electrodes supplied with high frequency alternating current from a suitable source such as an oscillator. The details of this cylinder are described hereinafter.

As the result of the heating effected by the electric field, solvent vapors are separated from the ink film. To facilitate removal of the vapors, a portion of the cylinder 1 I is enclosed by a hood 112. Air or any other suitable medium, preferably heated to a temperature which is above the dew'point of the highest boiling solvent employed and well above the web temperature, is introduced through an inlet l3 and escapes through an outlet i4 after passing in a direction counter to the direction of travel of the web 5. The web then passes over a cooling roller l5 which may be supplied with water or other cooling medium. The web may be withdrawn from the press or subjected to second side printing and subsequent drying in apparatus similar to that already described, as may be desired.

It is to be understood that the several cylinders and rollers are driven from any suitable source of power through the usual mechanism provided for that purpose (not shown). The details of the driving mechanism form no part of the present invention and are well understood in the art. Preferably the cylinder I I is driven at a peripheral speed slightly in excess. of the lineal speed of the web, as it is delivered thereto, in

. 4 i order to prevent any slack in the feed. This involves a slight slippage of the web with reference to the cylinder I i which is desirable because it ensures more uniform and complete action of the electric field in the heating of the film.

The cylinder II is illustrated in more detail in Figs. 2 and 3. It comprises end plates l6 of steel or other suitablematerial supported on hollow trio field between the electrodes.

trunnions ii and it. The end plates are connected by stay bolts i9 which are also of steel. At their outer periphery the end plates 96 support annular members 20 of insulating material preferably a ceramic material such as porcelain. The annular members iii carry respectively rings of conducting material 2! and 22 which in turn are connected to electrodes 23 and 24 arranged alternately about the periphery of the cylinder and supported by the insulating members 26. The alternate electrodes are thus electrically connected to the conducting rings at opposite ends of the cylinder. They are spaced from each other to afiord the maximum effective electric held when high frequency current is impressed on the electrodes. Preferably the electrodes are T- shaped, as shown in Fig. 4.

A coil '25 is fixedly supported within one end of the cylinder ii and connectedby conductors 2E and 27 to a source of high frequency current. Oscillators for producing such high frequency currents are well known in the art. and description thereof is unnec ssary.

A coil 23 is supported within the cylinder H on insulating plates 29 of porcelain or the like which are in turn mounted on bolts 39 secured to the stay bolts i9. Consequently the coil 28 rotates with the cylinder and the current induced by proximity of the coil 25 is available to supply the electrodes 23 and 2d. The ends of the coil 28 are connected by conductors 3i and 32 to the rings 2! and 22, thus ensuring the provision of the elec- In order to ensure rigidity, the electrodes 23 and 2d are supported between their ends by insulating rings 33 which are mounted on the stay bolt-s it. The end plates it, the insulating members 29, and the insulating rings 33 form a rotatable support for the electrodes 23 and 26.

As already indicated, it is desirable to maintain the emperature of the web during operation by applying heat to the under side thereof. This is accomplished by supporting on bolts 36 extending between the end plates iii a plurality of radiant heating units 35, preferably of the type adapted to provide infrared heat by the use of any suitable electric current. The details of mounting the heating units are well known in the art and need not be described. The current necessary to maintain the heating units in operation is supplied through conductors 3E and 31 connected respectively to rings 38 and 39 which are mounted on an insulating bracket 40 at one end of the cylinder H. The rings 38 and 39 are engaged by brushes 4| and 42 which in turn are connected by conductors 43 and 44 to a source of' current such as an ordinary AC or DC line. Through the use of the brushes, the cylinder H may rotate while maintaining the electrical connection. Other means for supplying heat to the under side of the web can be utilized as for example a supply of heated air which is circulated through the cylinder H.

In the operation of the apparatus as described, the web 5 passes continuously about the cylinder H which rotates preferably at a slightly higher peripheral speed than the linear speed of the web. The high frequency current delivered to the coil 25 induces a similar current in the coil 28, and the current flows through the circuit comprising the electrodes 23 and 24 and the gap between them, affording at the gap the electric field, unaccompanied by corona discharge, which is multiplied about the periphery of the cylinder between each pair of electrodes. This circuit is an. unbalanced one, as the charge induced at the opposite ends of coil 28 are of diiferent potential due to the ends of the coil being located in positions of different intensit within the field of coil 25. Thus the electrodes 23 and 24 have irnpressed on them charges'of different potential. As the web passes about the cylinder H. it is in actual contact with the electrodes during the period while the electric field is effective to produce heating in the ink film. The temperature of the ink film rises rapidly, with the result that vaporizable constituents are separated and are carried away preferably by the flow of air or other gas introduced to the hood 12. As the result of initial heating of the web, very little of the heat generated in the film escapes thereto. thus ensuring concentration of the heat produced by the electric field for effective use. Moreover, the radiant heat applied to the under side of the web helps to prevent the loss of heat from the film to the web. Extremely rapid operation is thus possible, that is to say, the linear speed of the web may be maintained at the maximum speed at which the drving can be effected by the use of the electric field.

In the drying of cloth, for instance, as d stinguished from the drying of a film applied thereto, there would be a system wherein the body of the web contains material which is active with respect to the field, i. e., water. In that case, the web itself would be heated necessarily by the effect of the electric field. Preheating and the application of radiant heat to the under side of the web would. however, supplement the action of the electric field and speed up the drying operation.

The method and apparatus as described afford a rapid and satisfactory procedure for the drying of films of various kinds. affording the maximum possible advantage of the heating effect of the electric field. Practical application of the in vention avoids many of the disadvantages of heating methods employing open flames or large steam drums and permits particularly more rapid drying than can be effected by any other means.

Various changes may be made in the details of the apparatus and in the method as described without departing from the invention or sacrificing the advantages thereof.

We claim:

1. An apparatus for drying a sheet, which comprises a rotatable cylinder, a plurality of pairs of insulated spaced electrodes carried on the peripheral surface of said cylinder, means for electrically connecting the opposed electrodes of each pair, and means for supplying high frequency alternating current to the opposed electrodes including a stationary coil axially projecting into one end of said cylinder and a second coil axially mounted within said cylinder and rotatable therewith, said second coil being connected to the electrical connecting means so that when said second coil is energized an electric field will be produced between the respective pairs of insulated spaced electrodes.

2. An apparatus for drying a relatively thin film deposited on one surface of a relatively thicker sheet which comprises a rotatable support, a plurality of pairs of spaced electrodes carried on the periphery of said support, means for electrically connecting the opposed electrodes of each pair, means for supplying high frequency alternating current to the opposed electrodes and means to advance said sheet over and in peripheral contact with said electrodes at a rate different from that at which the electrodes move so that the region of greatest intensity of the electric fields created by supplying the high frequency alternating current to said electrodes is caused to deviate from its normal path directly between said electrodes and to pass through the sheet in contact with them.

3. An apparatus for drying a relatively thin film deposited on one surface of a relatively thicker sheet as set forth in claim 2 which includes means to preheat said sheet to a temperature insufficient to effect appreciable drying of the thin film thereon.

4. An apparatus for drying a relatively thin film deposited on one surface of a relatively thicker sheet as set forth in claim 3, which includes separate heating means connected to and rotatable with said rotatable support to maintain the preheat of said sheet.

5. The method of drying a sheet which comprises maintaining a plurality of spaced electric fields, unaccompanied by corona discharge, between spaced pairs of electrodes connected to a source of high frequency alternating current, maintaining the sheet in peripheral contact with said electrodes, whereby the region of greatest intensity of said electric fields is caused to deviate from its normal path directly between said electrodes and to pass through said sheet, and causing relative movement between said sheet and said electrodes while the region of greatest intensity of said electric fields is passing through said sheet.

6. The method of drying a sheet which comprises maintaining a plurality of substantially parallel spaced electric fields, unaccompanied by corona discharge, between spaced pairs of electrodes connected to a source of high frequency alternating current, moving said electrodes and the resultant electric fields and advancing the sheet relatively to said electrodes over and in peripheral contact with them at a rate different from that at which the electrodes are moving, whereby the region of greatest intensity of said electric fields is caused to deviate from its normal path directly between said electrodes and to pass through said sheet.

7. The method of drying a sheet which comprises maintaining a plurality of spaced electric fields, unaccompanied by a corona discharge, between spaced pairs of electrodes connected to a source of high frequency alternating current, moving said electrodes and the resulting electric fields in a continuous path, and advancing the sheet over and in peripheral contact with said electrodes at a rate different from that at which the electrodes are moving, whereby the region of greatest intensity of said electric fields is caused to deviate from its normal path directly between said electrodes and to pass through said sheet.

8. The method of drying a sheet as set forth in claim 7, in which the electrodes and the resulting electric fields move in a continuous arcuate path, and in which the sheet is supported solely by said electrodes while it is passing in peripheral contact with said electrodes.

7 9. The method of drying a relatively thin film. deposited on one surface of a relatively thicker sheet which comprises preheating the sheet to a temperature insuilicient to effect appreciable drying of the film, maintaining a plurality of spaced electric fields, unaccompanied by corona discharge, between speed pairs of electrodes connected to a source of high frequency alternating current, maintaining the sheet in peripheral contact with said electrodes, whereby the region of greatest intensity of said electric fields is caused to deviate from its normal path directly between said electrodes and to pass through said sheet, and causing relative movement between said sheet and said electrodes while the region oi greatest intensity of said electric fields is passing through said sheet.

10. The method of drying a relatively thin film deposited on the surface of a relatively thicker sheet as set forth in claim 9, in which the preheat oi the sheet is separately maintained during the drying of the film thereon.

11. The method of drying a relatively thin film deposited on one surface of a relatively thicker sheet, as set forth in claim 9, in which the electric fields are moved in a continuous path.

12. The method of drying a relatively thin film deposited on one surface of a relatively thicker sheet as set forth in claim 11, in which the preheat of the sheet is separately maintained during the drying of the film thereon.

13. The method of drying a relatively thin film deposited on the surface of a relatively thicker sheet as set forth in claim 12, in which the electric fields are moved in a continuous arcuate path, and in which the sheet is supported solely by said electrodes while it is advancing in peripheral con tact with them.

lei. The method of drying a relatively thin film deposited on one surface of a relatively thicker sheet which comprises maintaining a plurality of substantially parallel, spaced electric fields,

unaccompanied by corona discharge, between alternate, substantially parallel, spaced electrodes connected, in succession, to the opposite poles of a source of high frequency alternating current which are at difierent potentials with respect to ground, maintaining the sheet in peripheral contact with said electrodes, whereby the region of greatest intensity of said electric fields is caused to deviate from its normal path directly between said electrodes and to pass through said sheet and film, and causing relative movement between said sheet and said electrodes while the region of greatest intensity of said electric fields is passing through said sheet and film.

15. An apparatus for drying relatively thin film deposited on one surface of a relatively thicker sheet which comprises a rotatable support, a plurality of pairs of spaced electrodes on the periphery of said support, means for electrically connecting the opposed electrodes of each pair, and means for impressing a high frequency alternating current on said electrodes including a coil secured within the support, said coil having its opposite ends connected to the meansfor electrically connecting the opposed electrodes of each pair, and a fixed coil connected to a source of high frequency alternating current, the coil secured within said support being so positioned with respect to said fixed coil that the opposite ends thereof are spaced at different distances therefrom and have induced in them from the fixed coil charges of different potential with respect to ground, whereby said successive electrodes receive charges of opposite polarity and diflerent potential with respect to ground and become part of an unbalanced electric circuit.

WALLACEP. COHOE. MITCHELL WILSON.

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